This invention relates to DC and AC inverters and, more particularly, to a highly efficient circuit which advantageously develops a sinusoidal waveform having a low harmonic content from rectangular or square wave components.
As discussed in the above-referenced copending application, inverters serve the purpose of providing AC power requirements when only DC is available. Inverters are well known and are usually considered to be off-the-shelf items when efficiency, weight and harmonic levels are not a consideration. However, when such considerations become important, one must carefully evaluate the particular inverter.
When evaluating inverters, several features are typically examined. Consideration is given to the total harmonic noise level rejection, the efficiency of the inverter and its total weight. An evaluation of these considerations is most critical where the environment of the inverter is adverse. For example, in aircraft, balloon or space use, it becomes important to obtain an inverter capable of providing power with a low harmonic content so that the noise resulting from the presence of the harmonics will not interfere with other airborne components and systems. Furthermore, good efficiency is a requirement because of the limited DC power available. Finally, the weight associated with the inverter is an important consideration for obvious reasons.
It is difficult to obtain a highly efficient, low harmonic, lightweight inverter, and usually the improvement of one characteristic is accomplished at the expense of one or both of the other two. As shown in the copending application, a sine wave can be approximated by generating a rectangular wave and then combining it with a stepped waveform while advantageously eliminating the first through the eleventh harmonics. The system disclosed in the copending application is desirable because it is light in weight and at the same time it has the capability of minimizing the harmonics.
I have now developed another inverter which eliminates the first through the eleventh harmonics, is even lighter in weight than the inverter disclosed in the copending application, and is driven by a fundamental rectangular component at a frequency equal to the synthesized sinusoidal output frequency for a maximization of the efficiency.